Back in 2003, I gave Joe a fond farewell at his retirement party. We talked about his travel plans and his well-considered retirement investments. I knew Joe had invested wisely and that he was careful with his money. So I was later surprised to see him back at work.

"It was an opportunity I couldn't resist," Joe explained. "The superintendent called and asked if I'd be interested in helping oversee some of the equipment repairs for about a month."

As an independent contractor, Joe remained blissfully free of office politics. He'd be on the job for only a month. After that, he was still headed to Yellowstone.

Several younger employees might have supervised the same repairs, but Joe had more experience with the unit in question than any of them did, and none of the new guys had dealt with such a costly outage. As part of the repair process, Joe helped train his successors. "I'm finally getting to do the fun part of the job," he told me, "without the headaches."

Getting back to work
I've known many people with stories like Joe's. When I was younger, I used to think these individuals, apparently having failed to plan and budget well, had to go back to work to make ends meet. But I discovered that in many cases, retired employees are simply a boon to their former employers. Let's look at the advantages for employers and retirees alike.

First, retirees have a lot of knowledge and skills to pass on to their successors. Besides, we old guys enjoy showing the newbies how smart we are!

Consider, too, that employee benefits can add 50% or more to companies' salary expenses. But since part-time employees aren't eligible for benefits, employers can bring back a retiree for the same wage he or she previously made, or an even higher one, and still save money. It's therefore not surprising that companies such as IBM (NYSE: IBM), Procter & Gamble (NYSE: PG), and Eli Lilly (NYSE: LLY) actively seek out retirees who want to come back to work on a flexible basis.

Besides, employers may need only a temporary employee. Most profitable companies operate lean and mean, lacking staff to handle anything but routine duties. Anything out of the ordinary may require extra personnel but only for a short time. That's a perfect opportunity for a retiree looking for a short stint back in the saddle.

Firsthand experience
As it turns out, I'm writing this article from personal experience. I retired from my job as a supervisor at Total Petrochemicals in June 2006. I truly loved my job, enjoyed my co-workers, and gained a lot of knowledge and experience in my particular field. However, a long and happy retirement was one of the goals for which I'd worked, so I welcomed retirement with open arms.

A few months later, I was invited back to help with an extensive maintenance outage in two of the units I'd supervised over the years. My job would be to help develop a shutdown plan and then serve as an advisor during the actual outage. The company wanted me to do the part of the job that I truly enjoyed, without the personnel issues that normally come with supervision! I'd be working for less than three months -- a perfect length for me. Even better, the shutdown came during our cold, rainy winter, so it didn't interfere with my summer plans. I loved it.

Then, another opportunity presented itself. The company's refinery operating procedures and manuals need constant updating, so I was offered a chance to work part-time revising those procedures. I'll be working about 10 days a month, mostly from home, and with flexible hours. I must be dreaming! I enjoy writing, and the company's willing to pay me a great deal of money to do it!

Icing on the cake
To a retiree, this is just like found money. If we've planned, as sensible Fools should, we don't need the money, or we wouldn't have retired in the first place. However, there's no such thing as having too much cash.

Few things make you feel quite as needed or appreciated as a former employer asking you to come back. If working part-time in retirement appeals to you, assess your skills. You probably acquired knowledge during your career that would be valuable to your former employer, or even to someone else in the same type of business.

If you think your skills are marketable, talk to your former employer. You may talk to a personnel search firm or a temp agency if you're really serious. These people are in business to match employer needs with employee skills. It's possible that somebody out there really wants what you have to offer.

Retirement doesn't just mean not working. It's an opportunity to enjoy yourself. And if you enjoy earning extra money and adding to your self-esteem, I'd say you're a pretty wise Fool.

Related Foolishness:

This article was originally published on Nov. 8, 2007. It has been updated by Dan Caplinger, who doesn't own shares of the companies mentioned in this article.

To take charge of your retirement, try out the Fool's Rule Your Retirement newsletter service. From making the right investments to deciding whether going back to work is right for you, you'll find an abundance of helpful advice. It's all there for you, free for 30 days, with no obligation.

Fool contributor Glen Kenney is enjoying his retirement. Eli Lilly is an Income Investor recommendation. The Motley Fool has a full-time disclosure policy.